Characterization of damage to photosystems I and II in a cyanobacterium lacking detectable iron superoxide dismutase activity.

Abstract

The enzyme superoxide dismutase is ubiquitous in aerobic organisms where it plays a major role in alleviating oxygen-radical toxicity. An insertion mutation introduced into the iron superoxide dismutase locus (designated sodE) of the cyanobacterium Synechococcus sp. PCC 7942 created a mutant strain devoid of detectable iron superoxide dismutase activity. Both wild-type and mutant strains exhibited similar photosynthetic activity and viability when grown with 17 jmolm-2-s'1 illumination in liquid culture supplemented with 3% carbon dioxide. In contrast, the sodB mutant exhibited significantly greater damage to its photosynthetic system than the wild-type strain when grown under increased oxygen tension or with methyl viologen. Although damage occurs at both photosystems I and II, it is primarily localized at photosystem I in the sodB mutant. Growth in 100% molecular oxygen for 24 hr decreased photoacoustically measured energy storage in 3-(3,4-dichlorophenyl)-1,1-dimethylurea and abolished the fluorescence state 2 to state 1 transition in the sodB mutant, indicating interruption of cyclic electron flow around photosystem I. Analysis of the flash-induced absorption transient at 705 nm indicated that the interruption of cyclic electron flow occurred in the return part of the cycle, between the two [4 Fe-4 SI centers of photosystem I, FA and FB, and cytochrome f. Even though the sodB mutant was more sensitive to damage by active oxygen than wild-type cells, both strains were equally sensitive to the photoinhibition of photosystem II caused by exposure to strong light.

Transcript Profiling Reveals New Insights into the Acclimation of the Mesophilic Fresh-Water CyanobacteriumSynechococcus elongatusPCC 7942 to Iron Starvation

“…In contrast, the mRNA level for a DnaJ-like protein of the HSP40 family was substantially decreased. The transcript concentration for the Fe superoxide dismutase (Herbert et al, 1992;Samson et al, 1994) was found to be lower in Fedepleted cells than in Fe-sufficient cells. Altogether, these findings reveal that Fe-independent detoxification systems compensate in part the Fe-dependent parts of the cellular detoxification system.…”

A Novel Gene, pmgA, Specifically Regulates Photosystem Stoichiometry in the CyanobacteriumSynechocystis Species PCC 6803 in Response to High Light1

“…Thus, the increased PSI content in the mutants may result in much more production of reactive species of oxygen, which might account for the loss of viability of the mutants under the prolonged stress of HL. It is widely accepted that electrons generated from PSI react with oxygen to produce the superoxide anions, which are mainly scavenged by superoxide dismutase and ascorbate peroxidase (Asada, 1992;Herbert et al, 1992). It was recently demonstrated that irreversible photoinhibition of PSI occurs at its acceptor side in chilling-sensitive plants under chilling stress, possibly due to a loss of protection against the reactive species of oxygen (Sonoike and Terashima, 1994;Sonoike, 1996).…”

A Heterocomplex of Iron Superoxide Dismutases Defends Chloroplast Nucleoids against Oxidative Stress and Is Essential for Chloroplast Development inArabidopsis

“…These results suggest that CSD2 is essential for protecting the chloroplasts in higher plants from photooxidative damage. In the unicellular cyanobacterium Synechococcus, which possesses two SODs, MnSOD in the thylakoids and FeSOD in the cytosol, a mutant strain lacking FeSOD is sensitive to photooxidative stress, and its PSI complex is inactivated (Herbert et al, 1992;Samson et al, 1994;Thomas et al, 1998). However, the role of FeSOD isozymes in maintaining chloroplast function has not yet been documented.…”